Root/kernel/user.c

1/*
2 * The "user cache".
3 *
4 * (C) Copyright 1991-2000 Linus Torvalds
5 *
6 * We have a per-user structure to keep track of how many
7 * processes, files etc the user has claimed, in order to be
8 * able to have per-user limits for system resources.
9 */
10
11#include <linux/init.h>
12#include <linux/sched.h>
13#include <linux/slab.h>
14#include <linux/bitops.h>
15#include <linux/key.h>
16#include <linux/interrupt.h>
17#include <linux/module.h>
18#include <linux/user_namespace.h>
19
20/*
21 * userns count is 1 for root user, 1 for init_uts_ns,
22 * and 1 for... ?
23 */
24struct user_namespace init_user_ns = {
25    .kref = {
26        .refcount = ATOMIC_INIT(3),
27    },
28    .creator = &root_user,
29};
30EXPORT_SYMBOL_GPL(init_user_ns);
31
32/*
33 * UID task count cache, to get fast user lookup in "alloc_uid"
34 * when changing user ID's (ie setuid() and friends).
35 */
36
37#define UIDHASH_MASK (UIDHASH_SZ - 1)
38#define __uidhashfn(uid) (((uid >> UIDHASH_BITS) + uid) & UIDHASH_MASK)
39#define uidhashentry(ns, uid) ((ns)->uidhash_table + __uidhashfn((uid)))
40
41static struct kmem_cache *uid_cachep;
42
43/*
44 * The uidhash_lock is mostly taken from process context, but it is
45 * occasionally also taken from softirq/tasklet context, when
46 * task-structs get RCU-freed. Hence all locking must be softirq-safe.
47 * But free_uid() is also called with local interrupts disabled, and running
48 * local_bh_enable() with local interrupts disabled is an error - we'll run
49 * softirq callbacks, and they can unconditionally enable interrupts, and
50 * the caller of free_uid() didn't expect that..
51 */
52static DEFINE_SPINLOCK(uidhash_lock);
53
54/* root_user.__count is 2, 1 for init task cred, 1 for init_user_ns->user_ns */
55struct user_struct root_user = {
56    .__count = ATOMIC_INIT(2),
57    .processes = ATOMIC_INIT(1),
58    .files = ATOMIC_INIT(0),
59    .sigpending = ATOMIC_INIT(0),
60    .locked_shm = 0,
61    .user_ns = &init_user_ns,
62};
63
64/*
65 * These routines must be called with the uidhash spinlock held!
66 */
67static void uid_hash_insert(struct user_struct *up, struct hlist_head *hashent)
68{
69    hlist_add_head(&up->uidhash_node, hashent);
70}
71
72static void uid_hash_remove(struct user_struct *up)
73{
74    hlist_del_init(&up->uidhash_node);
75    put_user_ns(up->user_ns);
76}
77
78static struct user_struct *uid_hash_find(uid_t uid, struct hlist_head *hashent)
79{
80    struct user_struct *user;
81    struct hlist_node *h;
82
83    hlist_for_each_entry(user, h, hashent, uidhash_node) {
84        if (user->uid == uid) {
85            atomic_inc(&user->__count);
86            return user;
87        }
88    }
89
90    return NULL;
91}
92
93/* IRQs are disabled and uidhash_lock is held upon function entry.
94 * IRQ state (as stored in flags) is restored and uidhash_lock released
95 * upon function exit.
96 */
97static void free_user(struct user_struct *up, unsigned long flags)
98    __releases(&uidhash_lock)
99{
100    uid_hash_remove(up);
101    spin_unlock_irqrestore(&uidhash_lock, flags);
102    key_put(up->uid_keyring);
103    key_put(up->session_keyring);
104    kmem_cache_free(uid_cachep, up);
105}
106
107/*
108 * Locate the user_struct for the passed UID. If found, take a ref on it. The
109 * caller must undo that ref with free_uid().
110 *
111 * If the user_struct could not be found, return NULL.
112 */
113struct user_struct *find_user(uid_t uid)
114{
115    struct user_struct *ret;
116    unsigned long flags;
117    struct user_namespace *ns = current_user_ns();
118
119    spin_lock_irqsave(&uidhash_lock, flags);
120    ret = uid_hash_find(uid, uidhashentry(ns, uid));
121    spin_unlock_irqrestore(&uidhash_lock, flags);
122    return ret;
123}
124
125void free_uid(struct user_struct *up)
126{
127    unsigned long flags;
128
129    if (!up)
130        return;
131
132    local_irq_save(flags);
133    if (atomic_dec_and_lock(&up->__count, &uidhash_lock))
134        free_user(up, flags);
135    else
136        local_irq_restore(flags);
137}
138
139struct user_struct *alloc_uid(struct user_namespace *ns, uid_t uid)
140{
141    struct hlist_head *hashent = uidhashentry(ns, uid);
142    struct user_struct *up, *new;
143
144    spin_lock_irq(&uidhash_lock);
145    up = uid_hash_find(uid, hashent);
146    spin_unlock_irq(&uidhash_lock);
147
148    if (!up) {
149        new = kmem_cache_zalloc(uid_cachep, GFP_KERNEL);
150        if (!new)
151            goto out_unlock;
152
153        new->uid = uid;
154        atomic_set(&new->__count, 1);
155
156        new->user_ns = get_user_ns(ns);
157
158        /*
159         * Before adding this, check whether we raced
160         * on adding the same user already..
161         */
162        spin_lock_irq(&uidhash_lock);
163        up = uid_hash_find(uid, hashent);
164        if (up) {
165            put_user_ns(ns);
166            key_put(new->uid_keyring);
167            key_put(new->session_keyring);
168            kmem_cache_free(uid_cachep, new);
169        } else {
170            uid_hash_insert(new, hashent);
171            up = new;
172        }
173        spin_unlock_irq(&uidhash_lock);
174    }
175
176    return up;
177
178out_unlock:
179    return NULL;
180}
181
182static int __init uid_cache_init(void)
183{
184    int n;
185
186    uid_cachep = kmem_cache_create("uid_cache", sizeof(struct user_struct),
187            0, SLAB_HWCACHE_ALIGN|SLAB_PANIC, NULL);
188
189    for(n = 0; n < UIDHASH_SZ; ++n)
190        INIT_HLIST_HEAD(init_user_ns.uidhash_table + n);
191
192    /* Insert the root user immediately (init already runs as root) */
193    spin_lock_irq(&uidhash_lock);
194    uid_hash_insert(&root_user, uidhashentry(&init_user_ns, 0));
195    spin_unlock_irq(&uidhash_lock);
196
197    return 0;
198}
199
200module_init(uid_cache_init);
201

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